Such a method and installation are known from document FR-A-2 658 119 (EP-A-O 442 836).
Although that known method gives full satisfaction with respect to the advantages it provides compared with earlier methods, it nevertheless turns out that receptacles obtained by implementing it have a bottom that is subject to deformation during hot-filling, with said deformation consisting in the bottom bulging outwardly, in particular in the zone where the bottom meets the side wall of the body. This bulging does not take place in regular manner around the periphery of the receptacle, so the shape of the bottom varies in uncontrolled manner, thereby causing the receptacle to be unstable; the bottom of the receptacle is also weakened in the zone where bulging is manifest; finally, the volume of the receptacle is modified thereby.
It turns out that bulging of the bottom is a result of excessive local stretching of the wall of the receptacle in the above-mentioned zone and of the stresses induced during final blowing of the hot-shrunk blank being released because of the heating caused by filling with a hot liquid.
For manifest reasons of feasibility, the hot-shrunk blank has transverse and longitudinal dimensions that are smaller than those of the cavity 5 of the mold in which final blowing takes place (see FIG. 1A of the accompanying drawings): this ensures that the blank is not pinched when the mold is closed. It has been shown that during final blowing of said blank under high pressure (e.g. in the range 3 to 4.times.10.sup.6 Pa), the top portion 6 of the body of the blank 3 is pressed in the first place against the walls of the mold cavity to form the shoulder of the final receptacle; thereafter the blank is pressed against the walls of the mold cavity in a manner that varies continuously from the shoulder towards the bottom, so that the bottom of the receptacle is formed last. Unfortunately, during this progressive molding process, forming of the shoulder followed by forming of the body of the receptacle means that their respective diameters are increased, and given that the thermoplastic material is not stretched from one region towards another, this is accompanied by a progressive decrease in the length of the blank. As a result, when it is formed, the not-yet formed bottom 7 of the blank is at a considerable distance from the bottom 8 of the mold cavity 5 (see FIG. 1B) so final forming of the bottom of the blank is accompanied by considerable stretching of the material along two axes (both longitudinal and transversal), which stretching is caused by expansion of the material: the receptacle 4 as finally obtained (see FIG. 1C) thus presents considerably greater stretching in its now-formed bottom zone 9, thus having reduced thickness, and it also has induced stresses which are greater than those in the remainder of the receptacle, which stresses will subsequently be released during heating of the kind caused by hot-filling.
Document EP-A-O 559 103 proposed a solution for remedying that drawback and obtaining a receptacle having a mechanically strong bottom. However, that known method is designed for manufacturing receptacles in which the central portion of the bottom is pushed back inwards, by means of a moving mold bottom, which deforms, by punching ("punting") the bottom of the hot-shrunk blank, which blank is shorter than the length of the final receptacle to be obtained, but longer than the distance between the orifice of the bottle neck and the top of the convex bottom of the final receptacle. In addition, operating that process of "punting" the bottom of the shrunk blank, and then blowing to form the final receptacle requires the presence of a rod inside the blank for centering it and holding it in place.
As a result, the known method described in that document is not simple to implement, mechanically speaking, and it is restricted to manufacturing receptacles having a re-entrant bottom, and that is not always required by users.